Conventionally, a converter capable of operating in a boundary mode may be a ringing choke converter (hereinafter abbreviated as RCC), FIG. 1 shows the circuit diagram of a standard RCC. As stated above, since the standard RCC operates in the boundary mode, when a transformer T1 of the RCC transfers its electric energy to a secondary winding thereof having an output voltage Vo, a primary winding of the transformer T1 has a voltage Vo·n where n is a ratio of the primary winding to the secondary winding. That is, a voltage VCE of a switch transistor Q1 is equal to a sum of an input voltage Vin and the voltage Vo·n of the primary winding (i.e., Vin+Vo·n). The electric energy is stored in a parasite capacitor of the circuit in a form of voltage.
In the above-mentioned conventional RCC, when the electric energy stored in the transformer T1 is not sufficient to conduct a diode D1 being in series connection to the secondary winding of the RCC, the diode D1 is cut off and a harmonic is generated by the parasite capacitor and inductance of the circuit. After that, if the switch transistor Q1 is not switched again, the voltage VCE of the switch transistor Q1 oscillates as a sine wave centered on Vin having an amplitude equal to Vo·n. The sine wave shows an exponential decrease due to the effect of impedance in the circuit. FIG. 2 shows a waveform graph of the RCC operated in the boundary mode, wherein the dash lines shows the sine wave oscillation of the voltage VCE and the voltage VCE of the switch transistor Q1 has a minimum value of Vin−Vo·n.
Thus, by appropriately designing a driver circuit of the switch transistor Q1 to drive the switch transistor Q1 when the voltage VCE of the switch transistor Q1 has a minimum value, switch loss of the switch transistor Q1 can be predicted through using the following equation.             Cs      ·                        (                      V            CE                    )                2              2    ·  fowhere CS is an equivalent stray capacitance of the circuit, and fo is an operating frequency of the switch transistor Q1. It is clear that the switch loss of the switch transistor Q1 will be reduced significantly as the voltage VCE of the switch transistor Q1 drops. However, since the RCC operates in the boundary mode, the operating frequency fo of the switch transistor Q1 will increase as the input voltage Vin increases and the output load decreases. Thus, according to the above equation for calculating the switch loss, the switch transistor Q1 will still generate a substantial switch loss. Hence, when the operating frequency fo increases, the switch loss will increase significantly.
In view of the above, in order to lower the switch loss to zero for substantially eliminating the problem occurred in a high frequency operating state when the typical RCC operates in the boundary mode, the following actions should be taken by the designers and manufacturers of converters in designing their control circuits:
(1) Parallelly coupling a diode to the collector and the emitter of the switch transistor Q1 of the RCC or replacing the switch transistor Q1 with a transistor having a parasite diode (e.g., metal-oxide-semiconductor field-effect transistor, abbreviated as MOSFET) such that the voltage VCE of the switch transistor Q1 can be clamped at a level by the diode or the parasite diode for performing a zero voltage switch after the harmonic has reached a zero voltage level.
(2) Designing the circuitry of the RCC such that the amplitude of the above sine wave can be equal to Vin and the feedback voltage of the primary winding become larger than Vin. As a result, the minimum value of voltage VCE of the switch transistor Q1 is zero, and a switch is made possible when the zero voltage level is reached.
However, the cost for taking the above actions is that a transistor capable of operating in a high voltage is required since there is 2·Vin voltage drop in the switch transistor Q1. Moreover, since the cost and impedance of the transistor are relatively high, taking the above actions will unfortunately not only increase the manufacturing cost of RCC, but also increase the conduction loss of the transistor. As an end, the total performance is low. Hence, it is desirable among designers and manufacturers of the art to devise a switch transistor Q1 of converter capable of performing a zero voltage switch under a variety of loads in a boundary mode without increasing the manufacturing cost and the conduction loss in order to overcome the above drawbacks of the prior art.